The present invention relates to a turbocharger and more particularly a variable displacement turbocharger.
Turbochargers, which serve to improve thermal efficiency of various types of prime movers, are driven by exhaust gases from the prime movers. The flow rate of exhaust gases from a prime mover varies depending upon a load on the prime mover so that conventionally part of the performance of the turbocharger is sacrificed. To overcome this problem, recently developed are variable displacement turbochargers in which the angle of nozzle blades can be varied depending upon a load on a prime mover so as to optimumly control a flow rate of exhaust gases flowing to a turbine wheel, whereby a high degree of thermal efficiency can be maintained all the time.
FIGS. 11 and 12 show a turbine unit of a variable displacement turbocharger of the type described above in which a shroud 3 is clamped between a turbine casing 1 and a gas outlet cover 2 and rotatably carries nozzle shafts 4 through bearings 5. A nozzle blade 6 is securely attached to one end of the nozzle shaft 4 on the side of the turbine casing 1 while a nozzle link 7 is attached to the other end of the nozzle shaft 4 on the side of the gas outlet cover 2.
A doughnut-shaped space 8 defined between the gas outlet cover 2 and the shroud 3 accomodates the nozzle links 7 as well as a nozzle driving ring 9 positioned and rotatably supported by projections of the bearings 5. The nozzle driving ring 9 is connected through intermediate links 10 to the nozzle links 7. Reference numeral 11 represents a turbine wheel.
Rotation of the nozzle driving ring 9 by an external drive source through a lever 13 causes the nozzle blades 6 to angle-displace through the nozzle links 7 by an angle corresponding to the angle of rotation of the nozzle driving ring 9.
Therefore when the nozzle driving ring 9 is rotated in response to the load on the prime mover, the angle of the nozzle blades 6 is changed to optimize a flow rate and an entering angle of the exhaust gases flowing into the turbine wheel 11, whereby the efficiency of the turbocharger can be improved.
In the variable displacement turbocharger of the type described above, the nozzle driving ring 9 is axially restrained by the nozzle links 7 and the intermediate links 10. This restraint is not so severe because of plays between the bearings 5 and nozzle shafts 4, between the intermediate links 10 and nozzle driving ring 9 and between the nozzle links 7 and the intermediate links 10. Therefore there is a fear that vibrations of the turbocharger may occur during the operation. The axial vibratory load of the relatively heavy nozzle driving ring 9 is received by these links, which may cause damage of connections between the links and wear of the sliding component parts.
High temperature exhaust gases flow through the turbine casing 1 so that the turbine casing 1 as well as the shroud 3 which is in intimate contact therewith and is partly exposed to the exhaust gases become high-temperatured. As the result, temperature of the bearings 5 on the shroud 3 also rises. The nozzle driving ring 9, which is supported by the bearings 5 as described above, is not directly exposed to the exhaust gases and is raised in temperature later than the turbine casing 1 and the shroud 3. As a result, positions of the bearings 5 may be deviated in the radial direction and inside the nozzle driving ring 9 which is not yet thermally expanded out so that the space between the inner surface of the nozzle driving ring 9 and the outer peripheral surfaces of the bearings 5 disappears, resulting in sticking between the nozzle driving ring 9 and the bearings 5.
Moreover, in the conventional turbocharger, the vertical weight (gravity) exerted to the nozzle driving ring 9 is received by a few of bearings 5 at their upper extremity positions through a line contact between the nozzle driving ring 9 and these bearings 5 so that the pressure loaded on these bearings 5 is considerably great. Therefore when the nozzle driving ring 9 is rotated to cause sliding contacts between the same and the bearings 5, wear therebetween tends to progress. If the sliding contact surfaces of the bearings 5 are worn, the center of the nozzle driving ring 9 is offset so that the angles of the nozzle blades depending upon the angle of rotation of the nozzle driving ring 9 become nonuniform. As a result, the turbocharger cannot exhibit a desired performance.
Furthermore, in the case of the angle-displaceable nozzle blade structure, when the opening angle of the nozzle blades is too much, the nozzle blades may damageably contact the turbine wheel. It follows therefore that control of the opening angle of the nozzles blades is needed for prevention of contact between the nozzles blades and the turbine wheel. In the turbocharger of the type described above, the opening angle is adjusted only on the side of driving the nozzle driving ring and no angle adjustment is made on the turbine side. Therefore when the angle control on the side of driving the nozzle driving ring cannot be performed properly, the nozzle blades are caused to open so widely that they contact the turbine blade, causing damages to both the nozzle blades and the turbine wheel.
In the case of the angle-displaceable nozzle blade structure described above, the smaller the gaps between the nozzle blades 6, the turbine casing 1 and the shroud 3, the higher the effect of guiding the exhaust gases by the nozzle blades 6 becomes so that the efficiency of the turbine can be improved. Especially a gap at the root of the nozzle blade 6 (that is, the gap between the nozzle blade 6 and the shroud 3) greatly influences the guide effect in comparison with a gap at the tip of the nozzle blade 6 (that is, the gap between the nozzle blade 6 and the turbine casing 1). It follows therefore that the gap at the root of the nozzle blade 6 must be reduced as much as possible, but actually the gap between the nozzle blade and the shroud cannot be extremely reduced because of the reasons described below.
The gap varies due to thermal deformations of the shroud and the nozzle blade. The gap variation is also caused by the oxidation-roughened surfaces of the shroud and nozzle blades and by the bent nozzle shaft 4 due to wear of the bearings 5. Therefore, with the root gap being made too small, the nozzle blade 6 may contact the shroud 3 to hinder smooth changing of the angle of the nozzle blade 6 or to cause sticking between the nozzle blade 6 and shroud 3, resulting in failure of changing the angle of the nozzle blade.
Furthermore, in the case of the above-described bearing structure, the nozzle shaft 4 has a throat 40 at its end adjacent to the nozzle blade 6. The throat 40 has a diameter which is greater than that of the remainder of the nozzle shaft 4 and is substantially equal to the outer diameter of the bearing 5.
The bearing 5 is fitted into the shroud 3 to abut on the throat 40 of the nozzle shaft 4 with all the flange surface of the throat 40 being made into contact with the end surface of the bearing 5.
With this structure, the nozzle blade 6 is generally urged by the pressure of the exhaust gases toward the shroud 3 so that wear friction between the nozzle shaft 4 and the bearing 5 is great. Overall wear friction due to the nozzle blades 6 is therefore unnegligibly great and greatly influences the capacity of the driving source for driving the nozzle blades. It becomes difficult to correctly control the angle of the nozzle blade since such high wear resistance will cause a hysterisis of the movement of the nozzle blade between the nozzle blade and a nozzle blade driving mechanism.
In order to ensure the sufficiently smooth motions of the moving parts, there must be provided a suitable gap between the nozzle shaft 4 and the bearing 5. This is disadvantageous in that the exhaust gases flow through the space between the nozzle shaft 4 and the bearing 5 into the space 8 and then leak through the space between a drive shaft 41 and the bush to the exterior.
The leakage of the exhaust gases causes contamination of the surrounding atmosphere and is dangerous because the temperature of the exhaust gases is high.
In view of the above, a primary object of the present invention is to restrain the nozzle driving ring in the axial direction effectively and with a high degree of accuracy.
A second object of the present invention is to ensure smooth rotation of the nozzle driving ring all the time without causing sticking, thereby varying the angle of the nozzle blade easily and accurately.
A third object of the present invention is to control maximum opening angle of the nozzle blade on the side thereof, thereby positively preventing the contact between the nozzle blade and the turbine wheel.
A fourth object of the present invention is to avoid contact between the nozzle blade and the shroud and to decrease wear friction between the nozzle blade and the bearing.
A fifth object of the present invention is to prevent leakage of exhaust gases from the mechanism for angle-displacing the nozzle blade.